Why is it that heat can cause a chemical change while cold can only cause a physical change? This question has been bothering me for a while.

Chris

11-19-2004, 05:18 PM

because only heat provides the energy needed to change the atomic structure et al? probably no help actually but physics was my thing anyway

Goki

11-19-2004, 05:34 PM

I'm not really sure but I guess what Chris said is right, though you can change the atomic structure of a matter with radioactive reactions too [and also number of electrons of a matter can be changed with redox reactions]. Heat is a form of enegry which is required to break the bonds between elements or to form new bonds. So that when you heat a matter, it has a chemical change.

Endymion

11-19-2004, 05:54 PM

"heat" is a form of energy. it's really a heat difference--which doesn't mean a tempeture difference--that can be used for work. this is done by encreasing the entropy (putting useful, heat-reservoir energy into random kinetic energy which can not be extracted for work). what do you mean by cold does physical change? like ice breaking open a rock? that's due to a change in the chemical structure of the water--crystalization.

heat TENDS to increase the rate at which chemical reactions occure because that is dependent on the gibbs free energy (mostly) which is H-TdS. if that value is less than zero, a reaction tends to be spontaneous. thus, a higher temperature tends to push this downward. H is the enthalpy, by the way.

Endymion

11-19-2004, 05:58 PM

to, um, expand upon what the guy above me said...

bonds have energy, to break a bond you have to put so much energy into it. the usual way to do this is to strike one of the atoms of the bond and transfer enough energy to it so that it breaks the bond. only kinetic energy can be transfered... mv^2/2 = 3/2kT where T is the temp, so at higher temps the atoms/molecules have more kinetic energy and so their bonds are broken at a higher rate (they're still broken when it's "cold", just that the rate might be on the order of one bond a second, whereas the reformation rate would be many times greater than that).

Chris

11-19-2004, 06:11 PM

ahhh its all coming back now, I did rather enjoy thermodynamics

greencows12

11-19-2004, 06:18 PM

you see, when two people fall in love, they transport "special"cells to an "egg", the mommy then has the baby.

JoY

11-19-2004, 07:19 PM

Edy probably explained it clearer, than I think I possibly could right now.

heat is a form of energy. you know how it goes, dear. molecules start moving faster & shit (kinetic energy), bump into each other, which can cause a possible reaction. it can catalyse a reaction, by lifting the amplitude of 'silent processes' to a treshold of the enquired energy, above which they actually become effective.
you'll understand you can't just let water react, you need different kinds of molecules, the right ones, combined. if not heat will only cause molecules to bump into each other & bounce away, & break the bonds, that keeps the existing molecules together.
(those chemistry classes go extremely far back, but were très cute. & yes, I realise I explained this in a rather retarded kiddy way, but it's the way I understood it best. I always need to picture something in the most simple way possible, when it comes to matters that aren't visible to me. that goes for anything)

cold causes molecules to move slower to the point of almost standing still. *shrugs* it just changes the state of matter logically this way.

it depends what you're heating though. if we're talking animal cells, enough heat can destruct proteins, in the sense that it hydrolyses them (H-bridges disappear) & detaches amino acids, thus changes the primaire structure of the cell.

I do hope I make at least some sort of sense at this time of the morning & that my post can remotely answer your question. if I've been bullshitting just now, please anyone notify me.

I'm tired.

Mota Boy

11-19-2004, 07:58 PM

what do you mean by cold does physical change? like ice breaking open a rock? that's due to a change in the chemical structure of the water--crystalization.

I was talking about the water itself freezing - a change in density.

And thank you, one and all; I'm annoyed to have forgotten viewing heat as energy. I just found chemistry excruciatingly boring until recently.

Betty

11-20-2004, 01:06 AM

I wouldn't say cold makes ice freeze, I would say it's lack of heat.

And I'm a chemist. And I hate thermodynamics.

RXP

11-20-2004, 01:09 AM

Someone 'splain something.

At 0K does everything, at least to the human eye dissapear? Because the particles, electrons, neutrons, whatever have lost all their K.E so stop orbiting the nucleas therefore, remain still and therefore disappear?

RXP

11-20-2004, 01:10 AM

you see, when two people fall in love, they transport "special"cells to an "egg", the mommy then has the baby.

Translated: when a female consents to having a cock shoved up her, she makes it shoot up liquid into her.

lousyskater

11-20-2004, 03:09 AM

Someone 'splain something.

At 0K does everything, at least to the human eye dissapear? Because the particles, electrons, neutrons, whatever have lost all their K.E so stop orbiting the nucleas therefore, remain still and therefore disappear?

being able to see something or not has nothing to do with it's Kinetetic Energy. when we see an object, we are not actually seeing the object. we are seeing the light that the object reflects. and depending on what colors the object absorbs and reflects will determine what color it will appear to the human eye. if the object reflects blue light, it will appear blue to the human eye.
________
LIVE SEX (http://livesexwebshows.com/)

Eccentric Sara

11-20-2004, 03:11 AM

"heat" is a form of energy. it's really a heat difference--which doesn't mean a tempeture difference--that can be used for work. this is done by encreasing the entropy (putting useful, heat-reservoir energy into random kinetic energy which can not be extracted for work). what do you mean by cold does physical change? like ice breaking open a rock? that's due to a change in the chemical structure of the water--crystalization.

heat TENDS to increase the rate at which chemical reactions occure because that is dependent on the gibbs free energy (mostly) which is H-TdS. if that value is less than zero, a reaction tends to be spontaneous. thus, a higher temperature tends to push this downward. H is the enthalpy, by the way.What he said!!!!! :D I was going to say something about heat energy increasing the rates of reactions and causing chemical changes but, er,he's already done that!!!!! Jeez,its such a long time since I've done chemistry!*shudders* Never again.

Endymion

11-20-2004, 05:19 AM

Someone 'splain something.

At 0K does everything, at least to the human eye dissapear? Because the particles, electrons, neutrons, whatever have lost all their K.E so stop orbiting the nucleas therefore, remain still and therefore disappear?

no, at 0K all motion does not stop. things are just at their minimum energy configuration. it's after five am and i just got back from a party, so i'm not going to go into it, but look up zero point energy. zero kinetic energy would violate various conditions found in quantum mechanics.

as for water freezing... <g> it has to do with both the PRESSURE and the TEMP. just look at a phase diagram. water has the fairly odd property of decreasing it's density when it freezes (ice I that is... most of the other forms have a higher density). most other substances have a positive slope along their solid/liquid phase space.